Implantable medical device voltage divider circuit for mitigating electromagnetic interference

ABSTRACT

An RF protection circuit mitigates potentially adverse effects that may otherwise result from electromagnetic interference (e.g., due to MRI scanning of a patient having an implanted medical device). The RF protection circuit may comprise a voltage divider that is deployed across a pair of cardiac electrodes that are coupled to internal circuitry of the implantable medical device. Each leg of the voltage divider may be referenced to a ground of the internal circuit, whereby the different legs are deployed in parallel across different circuits of the internal circuitry. In this way, when an EMI-induced (e.g., MRI-induced) signal appears across the cardiac electrodes, the voltages appearing across these circuits and the currents flowing through these circuits may be reduced. The RF protection circuit may be used in an implantable medical device that employs a relatively low capacitance feedthrough to reduce EMI-induced (e.g., MRI-induced) current flow in a cardiac lead.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/968,563, filed Dec. 15, 2010, entitled “Implantable Medical DeviceVoltage Divider Circuit for Mitigating Electromagnetic Interference,”which is incorporated herein by reference.

TECHNICAL FIELD

This application relates generally to implantable medical devices andmore specifically, but not exclusively, to circuitry for mitigatingeffects of electromagnetic interference (e.g., that may result from MRIscanning of a patient with an implanted medical device).

BACKGROUND

An implantable medical device may connect to one or more implantableconductors that are outside of the device. For example, an implantablecardiac rhythm management device (e.g., a pacemaker, a defibrillator, ora cardioverter) may connect to one or more leads implanted in or nearthe heart of a patient to monitor cardiac function and provide therapyfor a patient who suffers from cardiac arrhythmia. For example, theimplantable device may process signals received via implanted cardiacleads to track the type and timing of native cardiac signals. In theevent cardiac events are not occurring at appropriate times or undesiredcardiac events are detected, the implantable device may apply cardiacstimulation signals (e.g., pacing signals) to the heart via theimplanted cardiac leads in an attempt to restore normal cardiac rhythm.

Under certain circumstances, however, unintentional pacing may occurwhen a patient with an implantable medical device is subjected to strongelectromagnetic fields. For example, time-varying magnetic fieldsgenerated during magnetic resonance imaging (MRI) may induce currents inan implanted lead that may, in turn, stimulate (e.g., cause capture of)cardiac tissue. In some cases, this unintended pacing may cause cardiacfibrillation. Such MRI-induced currents may arise in different ways.

In some cases, pulsed magnetic gradients used during MRI scanning mayinduce voltage in an implanted cardiac lead connected to an implanteddevice. If such voltage appears across sufficiently low impedance, theresulting current flowing through the lead may cause stimulation of theheart.

In some cases, pulses of amplitude modulated radiofrequency (“RF”)energy from MRI scanning (e.g., with a carrier at 64 MHz or 128 MHz) mayenter the implantable medical device, whereupon the pulses are rectifiedby internal circuitry of the implantable medical device. The rectifiedsignal may then exit the implantable medical device as a lower frequencydemodulated signal on the implanted lead. This lower frequency signalmay then travel to the patient's heart via the implanted lead andpotentially cause unintended stimulation.

In view of the above, a physician may elect to not prescribe MRIscanning for a patient who has an implanted medical device.Consequently, such a patient may receive suboptimal treatment.Accordingly, a need exists for implantable medical devices that aresufficiently immune to the influence of MRI magnetic fields and otherelectromagnetic interference (EMI). This would enable, for example, apatient who has an implanted MRI-compatible medical device to have noextra restrictions going under an MRI scan as compared to a patient whodoes not have such an implanted device.

SUMMARY

A summary of several sample aspects of the disclosure follows. It shouldbe appreciated that this summary is provided for the convenience of thereader and does not wholly define the breadth of the disclosure. Forconvenience, one or more aspects of the disclosure may be referred toherein simply as “some aspects”.

The disclosure relates in some aspects to a protection circuit formitigating potential effects of EMI. For example, a protection circuitas taught herein may be used to mitigate potentially adverse effectsthat may otherwise be caused by signals that are generated as a resultof MRI scanning of a patient with an implantable medical device. In someaspects, the protection circuit reduces MRI-induced voltage acrossinternal circuitry of an implantable medical device during MRI scanning.By reducing MRI-induced voltage in this manner, the protection circuitprevents these signals from being rectified by the internal circuitryand then exiting the implantable medical device as lower frequencydemodulated signals that are capable of stimulating the heart.

In some aspects, a protection circuit as taught herein comprises acapacitive voltage divider that is deployed across cardiac electrodes(e.g., at least one cardiac lead electrode and, optionally, a caseelectrode) that are coupled to the internal circuitry. Here, each leg ofthe voltage divider may be referenced to a ground of the internalcircuitry, whereby the different legs are deployed in parallel acrossdifferent circuits of the internal circuitry. In this way, when anEMI-induced signal (e.g., an MRI-induced RF signal) appears across thecardiac electrodes, the voltages appearing across these circuits and thecurrents flowing through these circuits may be reduced (e.g., by up to afactor of 2). By reducing the voltages and currents in this manner, anycomponents of these circuits that have the capability of rectifying anEMI-induced signal appearing across the cardiac electrodes may beprevented from performing such rectification.

Advantageously, the capacitive voltage divider circuit operates only onRF signals without significantly affecting the cardiac signals that theinternal circuitry is intended to sense and without significantlyaffecting cardiac stimulation (e.g., pacing) signals intentionallygenerated by the internal circuitry. This is because the capacitorsprovide a low impedance at the high frequencies associated with, forexample, the MRI-induced signals and provide high impedance at the lowerfrequencies associated with cardiac pacing and/or cardiac sensing.

In some aspects, a capacitive voltage divider as taught herein may beused in conjunction with several input protection diodes to provideadditional reduction of RF voltage across internal circuitry. Here, eachleg of the capacitive voltage divider is placed in parallel with afast-switching diode. The diode is arranged such that it does not becomeforward-biased during normal device operation. For example, in anegative-ground system in which all input/output terminals have a higherpotential than ground, the anode of each diode is connected to ground.However, the capacitive voltage divider begins to operate in thepresence of an RF signal, raising the internal device ground potentialabove that of each input/output terminal for alternating ½ cycles. Ifthe voltage across either protection diode becomes large enough toforward-bias it, then the two diodes begin to conduct in a balancedfashion on alternate ½ cycles; effectively clipping the RF voltageacross each leg of the capacitive voltage divider and therefore acrossinternal circuitry to one forward-biased diode voltage drop.

In some aspects, the protection circuit may be used in conjunction witha feedthrough circuit in which capacitance has been reduced (e.g., to1.5 nanofarads from 4.7 nanofarads) in order to mitigate the hazard ofunwanted cardiac stimulation due to pulsed magnetic gradients duringMRI. Here, the lower capacitance value reduces current flow due toinduction by pulsed magnetic gradients in a cardiac lead coupled to theimplantable medical device. By reducing this current flow, thelikelihood that the current flow may cause unwanted cardiac stimulationis reduced. In practice, the use of a lower capacitance feedthroughcircuit may result in an increase in the RF energy injected into theinternal circuitry of the implantable medical device. However, bydeploying a protection circuit across the internal circuitry as taughtherein, this energy may be prevented from affecting the internalcircuitry.

An improved EMI filter (e.g., MRI filter) is thus provided through theuse of protection circuitry that is referenced to ground in accordancewith the teachings herein. For example, effective MRI-related filteringmay be achieved via a simpler design and with a lower component countthan may be achieved, for example, by a filter that is deployed inseries with the cardiac leads.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages will be more fullyunderstood when considered with respect to the following detaileddescription, the appended claims, and the accompanying drawings,wherein:

FIG. 1 is a simplified diagram of an embodiment of an implantablemedical device comprising a circuit that filters MRI-induced signals;

FIG. 2 is a simplified diagram illustrating sample internal circuitryfor an implantable medical device;

FIG. 3 is a simplified diagram illustrating sample equivalent circuitsfor the internal circuitry of FIG. 2;

FIG. 4 is a simplified diagram of a modification of FIG. 3 that includesan embodiment of a filter circuit for filtering MRI-induced signals;

FIGS. 5A and 5B depict sample rectified signals;

FIG. 6 is a simplified diagram illustrating a sample embodiment of acircuit for filtering MRI-induced signals;

FIG. 7 is a simplified diagram of an embodiment of an implantablemedical device that includes a substrate for one or more filtercircuits;

FIG. 8 is a simplified diagram of an embodiment of an implantablecardiac device in electrical communication with one or more leadsimplanted in a patient's heart for sensing conditions in the patient,delivering therapy to the patient, or providing some combinationthereof; and

FIG. 9 is a simplified functional block diagram of an embodiment of animplantable cardiac device, illustrating basic elements that may beconfigured to sense conditions in the patient, deliver therapy to thepatient, or provide some combination thereof.

In accordance with common practice the various features illustrated inthe drawings may not be drawn to scale. Accordingly, the dimensions ofthe various features may be arbitrarily expanded or reduced for clarity.In addition, some of the drawings may be simplified for clarity. Thus,the drawings may not depict all of the components of a given apparatusor method. Finally, like reference numerals may be used to denote likefeatures throughout the specification and figures.

DETAILED DESCRIPTION

The description that follows sets forth one or more illustrativeembodiments. It will be apparent that the teachings herein may beembodied in a wide variety of forms, some of which may appear to bequite different from those of the disclosed embodiments. Consequently,the specific structural and functional details disclosed herein aremerely representative and do not limit the scope of the disclosure. Forexample, based on the teachings herein one skilled in the art shouldappreciate that the various structural and functional details disclosedherein may be incorporated in an embodiment independently of any otherstructural or functional details. Thus, an apparatus may be implementedor a method practiced using any number of the structural or functionaldetails set forth in any disclosed embodiment(s). Also, an apparatus maybe implemented or a method practiced using other structural orfunctional details in addition to or other than the structural orfunctional details set forth in any disclosed embodiment(s).

FIG. 1 illustrates an embodiment of an implantable medical apparatus 100comprising an implantable medical device 102 configured for stimulatingcardiac tissue and/or sensing cardiac activity. The device 102 includesa voltage divider circuit 104 comprising capacitors 106 and 108. Thevoltage divider circuit 104 acts to prevent MRI-induced signals that mayenter the device 102 via an implantable cardiac lead 122 from beingrectified by a cardiac signal processing circuit 110 (e.g., cardiacpacing circuitry and/or cardiac sensing circuitry) of the device 102. Inthis way, the voltage divider circuit 104 may reduce the likelihood thatrectified signals will exit the device 102 via the cardiac lead 122 andstimulate cardiac tissue.

As discussed in more detail below, each leg (e.g., each capacitor 106 or108) of the voltage divider circuit 104 is provided in parallel acrosscorresponding circuitry (e.g., pacing and/or sensing circuit 112 or 114)of the cardiac signal processing circuit 110. Here, a center tap 116 ofthe voltage divider circuit 104 is coupled to a circuit ground 118 ofthe circuit 110. Accordingly, current that may otherwise flow into thecircuits 112 and 114 may instead flow through the voltage dividercircuit 104 to the circuit ground 118. Moreover, a lower overall loadimpedance (with respect to a “source” that generates the MRI-inducedcurrent) is provided by the placement of the voltage divider circuit 104in parallel with the circuit 110. Consequently, a lower voltage isinduced across the circuit 110, thereby reducing the likelihood that thecircuit 110 will output a rectified signal.

In practice, an implantable medical device will typically use afeedthrough capacitor (e.g., capacitor 120) to provide a first layer ofprotection against electromagnetic interference (EMI). For example, thefeedthrough capacitor may shunt high frequency current directly to thehousing (e.g., case electrode 128) of the device 102 and away frominternal circuitry of the device 102. Being physically located at thepoint where conductors enter the housing, the feedthrough capacitor mayshunt current away from the conductors at a point before the conductorsenter the housing, thereby reducing the likelihood that high frequencyenergy may re-radiate from the conductors within the housing.

In some implementations, a voltage divider circuit is employed in animplantable medical device that uses a feedthrough capacitor that hasrelatively low capacitance value (e.g., less than 2 nanofarads). Here,the duration of the MRI-induced current, and therefore also the amountof charge which may flow due to induction from pulsed magnetic fields inan implantable medical device, may be reduced by using smallerfeedthrough capacitors (i.e., by using a capacitor with a smallernominal capacitance value). For example, if the only path for charge (Q)is through the feedthrough capacitor, the reduction in charge isproportional to the reduction in capacitance (C) for a voltage step ofmagnitude V according to the relationship Q=CV.

The use of a lower capacitance value for the feedthrough capacitorresults in a decrease of the net charge which can flow due to a changein voltage in a current loop path including, in the example of FIG. 1, aconductor 130 of the cardiac lead 122, circuitry of the device 102coupled between terminals 124 and 126, and a return path through thecase electrode 128 of the device and through patient tissue back to thecardiac lead. In FIG. 1, the current path through the patient tissue isrepresented in a simplified manner by a dashed line 138. Consequently,current flow (e.g., due to pulsed gradient-induced voltage) through thispath is reduced, thereby reducing the likelihood that this MRI-inducedcurrent flow will stimulate cardiac tissue.

The use of a higher impedance feedthrough capacitor 120 would normallycause a higher RF voltage to appear across the circuit 110. However, asdiscussed above, the voltage divider circuit 104 reduces the amount ofMRI-induced RF voltage that appears across either circuit 112 or 114.Thus, the voltage divider circuit 104 compensates for adverse effectsthat may otherwise result from the use of a smaller feedthroughcapacitor.

Moreover, the voltage divider circuit 104 may prevent MRI-inducedcardiac stimulation without significantly affecting other operations ofthe device 102. For example, while the capacitors 106 and 108 provide alow impedance (e.g., 1-4 ohms) at frequencies (e.g., approximately 64MHz or 128 MHz) associated with MRI scanning, the capacitors 106 and 108provide a higher impedance (e.g., >1 Mohm) at frequencies associatedwith cardiac pacing and cardiac sensing (e.g., on the order of kilohertzor less). Consequently, pacing pulses output by the device 102 orcardiac signals to be sensed by the device 102 are not affected by thevoltage divider circuit 104.

FIG. 1 also illustrates that in some implementations the implantablemedical device 102 may include input protection diodes 134 and 136(e.g., fast-switching diodes). In accordance with the teachings herein,a capacitive voltage divider may be used in conjunction with the diodes134 and 136 to provide additional reduction of RF voltage across thecircuit 110. Here, the capacitor 106 is in parallel with the diode 134and the capacitor 108 is in parallel with the diode 136. The diodes 134and 136 are arranged so that they are not forward-biased during normaldevice operation. For example, in a negative-ground system whereinput/output terminals (corresponding to the electrode circuits 124 and128) of the device 102 have a higher potential than the ground circuit118, the anodes of the diodes 134 and 136 are coupled to the groundcircuit 118 as shown in FIG. 1. In the presence of an RF signal,however, the capacitive voltage divider (capacitors 106 and 108) beginsto operate, thereby raising the internal device ground potential (groundcircuit 118) above that of each input/output terminal for alternating ½cycles. If the voltage across either diode 134 or 136 becomes largeenough to forward-bias the diode, the two diodes 134 and 136 begin toconduct in a balanced fashion on alternate ½ cycles; effectivelyclipping the RF voltage across each leg of the capacitive voltagedivider (and therefore across each circuit 112 and 114) to oneforward-biased diode voltage drop.

A capacitive voltage divider as taught herein may be implemented withvarious types of electrode circuits. In the example of FIG. 1, theelectrode circuit 124 is electrically coupled to an electrode 132 (e.g.,a tip or ring circuit) of the cardiac lead 122 while the electrodecircuit 126 is electrically coupled to the case electrode 128. Otherimplementations, however, may employ multiple cardiac leads or cardiacleads with multiple electrodes. For example, the electrode circuit 126may be electrically coupled to an electrode in another implantablecardiac lead (not shown). Alternatively, the electrode circuits 124 and126 may be electrically coupled to electrodes (e.g., bi-polar tip andring electrodes) of a single cardiac lead.

With the above overview in mind, additional details regarding how acapacitive voltage divider as taught herein may be employed to preventMRI-induced cardiac stimulation will be described with reference toFIGS. 2-4. For purposes of illustration these figures describe anexternal interface (e.g., an input/output stage) of a cardiacstimulation device that includes a sample implementation of a pacingoutput stage. To reduce the complexity of these figures, other circuitry(e.g., sensing circuitry and/or other pacing circuitry) that may beemployed in conjunction with the external interface is not shown. Inaddition, it should be appreciated that the teachings herein areapplicable to other types of circuits.

FIG. 2 depicts a cardiac stimulation device 200 including a sampleimplementation of a pacing output stage. Electrode circuits 202 and 204are configured to couple the pacing output stage to external electrodes(e.g., a cardiac lead electrode or a case electrode, not shown in FIG.2). A feedthrough capacitor 206 is employed across the electrodecircuits 202 and 204 as described herein. The pacing output stageincludes a pacing series capacitor 208 (e.g., 4.7 microfarads or someother suitable value), a return switch 210 and associated returnresistor 212 (e.g., 7.5 ohms or some other suitable value), and a fastdischarge switch 214. The pacing output stage circuits here arereferenced to a circuit ground 216 (e.g., device ground). The inputs ofthe device 200 (i.e., the electrode circuits 202 and 204) are notdirectly referenced to the circuit ground 216, but are insteadreferenced to the circuit ground 216 via the pacing output stagecircuitry.

FIG. 3 illustrates the pacing output stage of FIG. 2 during a fastrecharge operation (e.g., during left ventricle fast recharge) showingequivalent circuit 302 and 304 for the return switch 210 and the fastdischarge switch 214, respectively. When a pacing pulse is not beingapplied to a patient (e.g., during recharge), the return switch 210 isturned off (i.e., the switch is “open”). In contrast, the equivalentcircuit 304 for the fast discharge switch may have an effectiveresistance on the order of 40 ohms.

FIG. 3 also illustrates a hypothetical signal source 318 and ahypothetical source impedance 320 that may be associated with thegeneration of an MRI-induced signal that is injected into the device 200via the electrode circuits 202 and 204 (e.g., via a cardiac lead coupledto the device 200). As discussed herein, pulsed magnetic gradients usedduring MRI scanning may induce current in a “circuit” consisting of thelead, the device and patient tissue.

Under certain circumstances it is possible that rectification of thisMRI-induced signal may occur within the pacing output circuitry (e.g.,via ground-referenced transistors or diodes). For example, as shown inthe equivalent circuits 302 and 304, parasitic diodes of the fieldeffect transistors (FETs) in the pacing output stage circuitry may causerectification of incoming RF current. Accordingly, a rectified signalmay exit the device 200 as represented by the current flow arrows ofFIG. 3.

FIG. 4 depicts an embodiment of an external interface of a cardiacstimulation device 400 that illustrates how a voltage divider as taughtherein may be employed in the circuit of FIG. 3. In FIG. 4, electrodecircuits 402 and 404, feedthrough capacitor 406, pacing series capacitor408, equivalent return switch circuit 410, return resistor 412,equivalent fast discharge switch circuit 414, and circuit ground 416correspond to similar components of FIG. 3. In this case, however, thefeedthrough capacitor 406 may have a lower capacitance value than thefeedthrough capacitor 206 (e.g., 1.5 nanofarads instead of 4.7nanofarads) to reduce MRI-induced current in an implantable cardiac lead(not shown in FIG. 4) that may be coupled to the device 400.

The voltage divider is implemented in FIG. 4 by placing a firstcapacitor 422 between the electrode circuit 404 and the circuit ground416 and a second capacitor 424 between the electrode circuit 402 and thecircuit ground 416. The capacitor 422 (e.g., corresponding to capacitor108 in FIG. 1) is in parallel with the circuitry including the fastdischarge switch. Thus, the fast discharge switch circuit (asrepresented by the equivalent circuit 414) may correspond to the circuit114 of FIG. 1. The capacitor 424 (e.g., corresponding to capacitor 106in FIG. 1) is in parallel with the circuitry including the returnswitch. Thus, this circuitry (as represented by the capacitor 408, theequivalent circuit 410, and the return resistor 412) may correspond tothe circuit 112 of FIG. 1.

At MRI-related frequencies, the ground-referenced capacitors 422 and 424act as a low impedance divider across the diodes illustrated in FIG. 4.For example, in some implementations the capacitors 422 and 424 may eachhave a capacitance value in the range of 500 picofarads to 5 nanofarads.Consequently, each capacitor may have an impedance on the order of 5 to0.5 ohms at 64 MHz or 2.5 to 0.25 ohms at 128 MHz.

Any MRI-induced voltage across the two circuits of the fast dischargestage is decreased due to the capacitive voltage divider effect. Forexample, the voltage divider is configured so that approximately half ofthe voltage appearing across the feedthrough capacitor appears acrossthe fast discharge switch circuitry while the rest of this voltageappears across the return switch circuitry. That is, the voltage dividermay tend to hold the device ground at a voltage that is halfway betweenthe voltage at the electrode, circuit 402 and the voltage at theelectrode circuit 404. In contrast, in the absence of the voltagedivider, the fast discharge switch (in its low impedance state) will tryto force the full voltage appearing across the feedthrough capacitor toappear across the return switch (operating in an open state). In thatcase, rectification by the parasitic diode of the return switch mayoccur if the RF voltage across the return switch is high enough.

Advantageously, the voltage divider reduces the MRI-induced voltage thatappears across the return switch, thereby reducing the likelihood thatsuch rectification may occur. In an implementation where the capacitors422 and 424 are each 500 picofarads, the voltage divider places 5 ohmsin parallel with return switch and with the 40 ohm resistance of thefast discharge switch at a frequency of 64 MHz. If there were 5 ohms inboth legs of the voltage divider, then the RF voltage would beeffectively split in half at ground, and voltage across the returnswitch would be effectively minimized. In the example of FIG. 4,however, the 40 ohm resistance of the fast discharge switch reduces theeffective impedance of that leg of the voltage divider, causing slightlymore than half the RF voltage to appear across the return switch. If thefast return switch had a lower “on” resistance, then more RF voltagewould appear across the return switch. Thus, the values of thecapacitors making up the voltage divider may be chosen such that: 1) theimpedance of the capacitors at relevant frequencies is less than thelowest impedance of the circuitry that the capacitors bypass (e.g.,their impedance may be specified to be no more than 22% of the impedanceof the circuitry they bypass in order to keep ground within about 10% ofthe half-way point); and 2) the series combination of these capacitorsin parallel with the feedthrough capacitor maintains sufficient controlof gradient-induced current pulse width. The first consideration wouldhave the voltage divider capacitor values be as large as possible, whilethe second consideration would have the voltage divider capacitor valuesbe as small as possible. Therefore a design trade off is involved. Itshould be noted also that the optional protection diodes, such asdescribed in FIG. 1 (134 and 136) would appear in parallel with thecapacitors (422 and 424) making up the voltage divider.

Through the use of protection circuitry as taught herein, a significantreduction in rectified RF current may be achieved. For example, FIGS. 5Aand 5B illustrate sample rectified RF signals in a case where a voltagedivider is not used (FIG. 5A) and in a case where 500 picofaradcapacitors are provided between each electrode circuit and ground (FIG.5B). In this example, there is nearly 1 milliamp of rectified current502 when the voltage divider is not used, and considerably lessrectified RF current 504 when the voltage divider is used (1 milliampcorresponds to approximately 2 volts in FIGS. 5A and 5B). In general, itmay be desirable to configure the voltage divider circuit to limit therectified current to a magnitude on the order of 50-200 microamps orless (e.g., as shown in FIG. 5B) to prevent potential cardiacstimulation.

Advantageously, this reduction in rectified current may be achievedwithout significantly increasing currents resulting from pulsedgradients during MRI. As mentioned above, the addition of voltagedivider capacitors in parallel with the feedthrough capacitor mayincrease the effective capacitance across the input of the implantablemedical device. Nevertheless, the induced gradients that occur when avoltage divider is used (e.g., with 500 picofarads capacitors) may besubstantially equal to the induced gradients that occur when a voltagedivider is not used (e.g., in an implementation that uses a 1.5nanofarad feedthrough capacitor).

A voltage divider as taught herein may be employed with other MRIfiltering schemes. For example, FIG. 6 illustrates an implementation ofan implantable medical apparatus 600 that employs resonant filtercircuits 602 for reducing MRI-induced signals. In this example, aninductor-capacitor (LC) tank circuit 604 is provided in series with animplantable cardiac lead 606 coupled to a first terminal 608 of acardiac sensing and/or pacing circuit 610, while a series LC circuit 612is shunted across the first terminal 608 and a second terminal 614 ofthe circuit 610.

The LC circuits are configured to attenuate MRI-induced signals. Forexample, the LC tank circuit may have high impedance at MRI-relatedfrequencies while the series LC circuit may have low impedance atMRI-related frequencies. In this way, any signals induced in the cardiaclead 606 by MRI scanning may be significantly attenuated before theyenter the circuit 610.

To this end, the resonant filter circuits may be tuned to a frequencyassociated with MRI scanning (e.g., in the 64 MHz range, the 128 MHzrange, some other frequency range, or a combination of these ranges).For example, both circuits 604 and 612 may have a resonant frequency of64 MHz or 128 MHz. Alternatively, one of these circuits may have aresonant frequency of 64 MHz, while the other circuit has a resonantfrequency of 128 MHz.

A feedthrough capacitor 616 is provided across electrode circuits 618and 620. In some implementations, the feedthrough capacitor 616 may havea relatively low capacitance value for reducing MRI gradient currents asdiscussed above. In such a case, the LC tank circuit 604 and the seriesLC circuit 612 may serve to attenuate MRI-induced RF energy and tocompensate for any potential degradation in EMI protection that may becaused by the use of a low nominal capacitance value for the feedthroughcapacitor 616.

Voltage divider capacitors 622 and 624 are provided across the first andsecond terminals 608 and 614 of the circuit 610. As discussed herein, acenter tap of the voltage divider is coupled to a ground circuit 626 ofthe circuit 610 to prevent rectification of MRI-induced signals. Thevoltage divider may thus be used in conjunction with the resonant filtercircuits to further reduce the magnitude of the MRI-induced signals thatenter the circuit 610.

Other resonant circuit configurations may be employed in otherembodiments. For example, multiple LC tank circuits may be used insteadof a single LC tank circuit and/or multiple series LC circuits may beused instead of a single series LC circuit. Also, multiple sets of LCtank and series LC circuits may be employed instead the single set shownin FIG. 6. For example, another set of LC tank and series LC circuitsmay be employed between the series LC circuit 612 and the voltagedivider capacitors 622 and 624. In addition, input protection diodes maybe employed in the example of FIG. 6 to provide additional reduction ofRF voltage across internal circuitry. As described above, each leg ofthe capacitive voltage divider may be placed in parallel with afast-switching diode. For example, a diode (e.g., corresponding to thediode 134 of FIG. 1) may be employed across the first terminal 608 andthe ground circuit 626, and a diode (e.g., corresponding to the diode136 of FIG. 1) may be employed across the second terminal 614 and theground circuit 626.

FIG. 7 illustrates a sample implantable medical device 700 where atleast a portion of the filter circuitry as taught herein is implementedon a substrate (e.g., on a printed circuit board) 702 placed immediatelyafter a feedthrough 704 of the implantable medical device 700. Here, thefeedthrough 704 is hermetically sealed to a conductive, biocompatiblehousing 706 of the device 700, and also provides a hermetically sealedpassage for one or more conductors 708 that enable signals be coupledbetween an internal circuit 710 and an external connector 712 (providedwithin a header 714) of the device 700. The substrate 702 may be mountedonto the feedthrough 704 so that the filter circuits are as close aspossible to the feedthrough to reduce the amount of stray RF energy thatmay be radiated via the conductor(s) 708 inside the housing 706.

Various components may be provided on the substrate 702. For example, insome implementations at least a portion of the feedthrough capacitancemay be implemented on the substrate 702 (e.g., instead of within thefeedthrough 704). In some implementations one or more filter circuits(e.g., one or more of filter circuits 106, 108, 422, 424, 604, 612, 622,or 624) may be implemented on the substrate 702.

In practice, filter circuitry for multiple channels (for multiplecardiac leads) may be implemented on the substrate 702. Given theexcellent current and voltage attenuation that may be achieved by thisfilter circuitry, crosstalk interference may be negligible even when arelatively small substrate having minimal PCB trace clearance isemployed. It should be noted that any inductors employed in this designmay have non-magnetic cores. Magnetic cores would be likely to saturatein the strong MRI static field, drastically altering inductance values.

FIGS. 8 and 9 describe an exemplary implantable medical device (e.g., astimulation device such as a pacemaker, an implantable cardioverterdefibrillator, etc.) that is capable of being used in connection withthe various embodiments that are described herein. It is to beappreciated and understood that other devices, including those that arenot necessarily implantable, can be used and that the description belowis given, in its specific context, to assist the reader inunderstanding, with more clarity, the embodiments described herein.

FIG. 8 shows an exemplary implantable cardiac device 800 in electricalcommunication with a patient's heart H by way of three leads 804, 806,and 808, suitable for delivering multi-chamber stimulation and shocktherapy. Bodies of the leads 804, 806, and 808 may be formed ofsilicone, polyurethane, plastic, or similar biocompatible materials tofacilitate implant within a patient. Each lead includes one or moreconductors, each of which may couple one or more electrodes incorporatedinto the lead to a connector on the proximal end of the lead. Eachconnector, in turn, is configured to couple with a complimentaryconnector (e.g., implemented within a header) of the device 800.

To sense atrial cardiac signals and to provide right atrial chamberstimulation therapy, the device 800 is coupled to an implantable rightatrial lead 804 having, for example, an atrial tip electrode 820, whichtypically is implanted in the patient's right atrial appendage orseptum. FIG. 8 also shows the right atrial lead 804 as having anoptional atrial ring electrode 821.

To sense left atrial and ventricular cardiac signals and to provide leftchamber pacing therapy, the device 800 is coupled to a coronary sinuslead 806 designed for placement in the coronary sinus region via thecoronary sinus for positioning one or more electrodes adjacent to theleft ventricle, one or more electrodes adjacent to the left atrium, orboth. As used herein, the phrase “coronary sinus region” refers to thevasculature of the left ventricle, including any portion of the coronarysinus, the great cardiac vein, the left marginal vein, the leftposterior ventricular vein, the middle cardiac vein, the small cardiacvein or any other cardiac vein accessible by the coronary sinus.

Accordingly, an exemplary coronary sinus lead 806 is designed to receiveatrial and ventricular cardiac signals and to deliver left ventricularpacing therapy using, for example, a left ventricular tip electrode 822and, optionally, a left ventricular ring electrode 823; provide leftatrial pacing therapy using, for example, a left atrial ring electrode824; and provide shocking therapy using, for example, a left atrial coilelectrode 826 (or other electrode capable of delivering a shock). For amore detailed description of a coronary sinus lead, the reader isdirected to U.S. Pat. No. 5,466,254, “Coronary Sinus Lead with AtrialSensing Capability” (Helland), which is incorporated herein byreference.

The device 800 is also shown in electrical communication with thepatient's heart H by way of an implantable right ventricular lead 808having, in this implementation, a right ventricular tip electrode 828, aright ventricular ring electrode 830, a right ventricular (RV) coilelectrode 832 (or other electrode capable of delivering a shock), and asuperior vena cava (SVC) coil electrode 834 (or other electrode capableof delivering a shock). Typically, the right ventricular lead 808 istransvenously inserted into the heart H to place the right ventriculartip electrode 828 in the right ventricular apex so that the RV coilelectrode 832 will be positioned in the right ventricle and the SVC coilelectrode 834 will be positioned in the superior vena cava. Accordingly,the right ventricular lead 808 is capable of sensing or receivingcardiac signals, and delivering stimulation in the form of pacing andshock therapy to the right ventricle.

The device 800 is also shown in electrical communication with a lead 810including one or more components 844 such as a physiologic sensor. Thecomponent 844 may be positioned in, near or remote from the heart.

It should be appreciated that the device 800 may connect to leads otherthan those specifically shown. In addition, the leads connected to thedevice 800 may include components other than those specifically shown.For example, a lead may include other types of electrodes, sensors ordevices that serve to otherwise interact with a patient or thesurroundings.

FIG. 9 depicts an exemplary, simplified block diagram illustratingsample components of the device 800. The device 800 may be adapted totreat both fast and slow arrhythmias with stimulation therapy, includingcardioversion, defibrillation, and pacing stimulation. While aparticular multi-chamber device is shown, it is to be appreciated andunderstood that this is done for illustration purposes. Thus, thetechniques and methods described below can be implemented in connectionwith any suitably configured or configurable device. Accordingly, one ofskill in the art could readily duplicate, eliminate, or disable theappropriate circuitry in any desired combination to provide a devicecapable of treating the appropriate chamber(s) with, for example,cardioversion, defibrillation, and pacing stimulation.

A housing 900 for the device 800 is often referred to as the “can”,“case” or “case electrode”, and may be programmably selected to act asthe return electrode for all “unipolar” modes. The housing 900 mayfurther be used as a return electrode alone or in combination with oneor more of the coil electrodes 826, 832 and 834 for shocking purposes.The housing 900 may be constructed of a biocompatible material (e.g.,titanium) to facilitate implant within a patient.

The housing 900 further includes a connector (not shown) having aplurality of terminals 901, 902, 904, 905, 906, 908, 912, 914, 916 and918 (shown schematically and, for convenience, the names of theelectrodes to which they are connected are shown next to the terminals).The connector may be configured to include various other terminals(e.g., terminal 921 coupled to a sensor or some other component)depending on the requirements of a given application.

To achieve right atrial sensing and pacing, the connector includes, forexample, a right atrial tip terminal (AR TIP) 902 adapted for connectionto the right atrial tip electrode 820. A right atrial ring terminal (ARRING) 901 may also be included and adapted for connection to the rightatrial ring electrode 821. To achieve left chamber sensing, pacing, andshocking, the connector includes, for example, a left ventricular tipterminal (VL TIP) 904, a left ventricular ring terminal (VL RING) 905, aleft atrial ring terminal (AL RING) 906, and a left atrial shockingterminal (AL COIL) 908, which are adapted for connection to the leftventricular tip electrode 822, the left ventricular ring electrode 823,the left atrial ring electrode 824, and the left atrial coil electrode826, respectively.

To support right chamber sensing, pacing, and shocking, the connectorfurther includes a right ventricular tip terminal (VR TIP) 912, a rightventricular ring terminal (VR RING) 914, a right ventricular shockingterminal (RV COIL) 916, and a superior vena cava shocking terminal (SVCCOIL) 918, which are adapted for connection to the right ventricular tipelectrode 828, the right ventricular ring electrode 830, the RV coilelectrode 832, and the SVC coil electrode 834, respectively.

At the core of the device 800 is a programmable microcontroller 920 thatcontrols the various modes of stimulation therapy. As is well known inthe art, microcontroller 920 typically includes a microprocessor, orequivalent control circuitry, designed specifically for controlling thedelivery of stimulation therapy, and may further include memory such asRAM, ROM and flash memory, logic and timing circuitry, state machinecircuitry, and I/O circuitry. Typically, microcontroller 920 includesthe ability to process or monitor input signals (data or information) ascontrolled by a program code stored in a designated block of memory. Thetype of microcontroller is not critical to the describedimplementations. Rather, any suitable microcontroller 920 may be usedthat carries out the functions described herein. The use ofmicroprocessor-based control circuits for performing timing and dataanalysis functions are well known in the art.

Representative types of control circuitry that may be used in connectionwith the described embodiments can include the microprocessor-basedcontrol system of U.S. Pat. No. 4,940,052 (Mann et al.), thestate-machine of U.S. Pat. No. 4,712,555 (Thornander et al.) and U.S.Pat. No. 4,944,298 (Sholder), all of which are incorporated by referenceherein. For a more detailed description of the various timing intervalsthat may be used within the device and their inter-relationship, seeU.S. Pat. No. 4,788,980 (Mann et al.), also incorporated herein byreference.

FIG. 9 also shows an atrial pulse generator 922 and a ventricular pulsegenerator 924 that generate pacing stimulation pulses for delivery bythe right atrial lead 804, the coronary sinus lead 806, the rightventricular lead 808, or some combination of these leads via anelectrode configuration switch 926. It is understood that in order toprovide stimulation therapy in each of the four chambers of the heart,the atrial and ventricular pulse generators 922 and 924 may includededicated, independent pulse generators, multiplexed pulse generators,or shared pulse generators. The pulse generators 922 and 924 arecontrolled by the microcontroller 920 via appropriate control signals928 and 930, respectively, to trigger or inhibit the stimulation pulses.

Microcontroller 920 further includes timing control circuitry 932 tocontrol the timing of the stimulation pulses (e.g., pacing rate,atrio-ventricular (A-V) delay, atrial interconduction (A-A) delay, orventricular interconduction (V-V) delay, etc.) or other operations, aswell as to keep track of the timing of refractory periods, blankingintervals, noise detection windows, evoked response windows, alertintervals, marker channel timing, etc., as known in the art.

Microcontroller 920 further includes an arrhythmia detector 934. Thearrhythmia detector 934 may be utilized by the device 800 fordetermining desirable times to administer various therapies. Thearrhythmia detector 934 may be implemented, for example, in hardware aspart of the microcontroller 920, or as software/firmware instructionsprogrammed into the device 800 and executed on the microcontroller 920during certain modes of operation.

Microcontroller 920 may include a morphology discrimination module 936,a capture detection module 937 and an auto sensing module 938. Thesemodules are optionally used to implement various exemplary recognitionalgorithms or methods. The aforementioned components may be implemented,for example, in hardware as part of the microcontroller 920, or assoftware/firmware instructions programmed into the device 800 andexecuted on the microcontroller 920 during certain modes of operation.

The electrode configuration switch 926 includes a plurality of switchesfor connecting the desired terminals (e.g., that are connected toelectrodes, coils, sensors, etc.) to the appropriate I/O circuits,thereby providing complete terminal and, hence, electrodeprogrammability. Accordingly, switch 926, in response to a controlsignal 942 from the microcontroller 920, may be used to determine thepolarity of the stimulation pulses (e.g., unipolar, bipolar, combipolar,etc.) by selectively closing the appropriate combination of switches(not shown) as is known in the art.

Atrial sensing circuits (ATR. SENSE) 944 and ventricular sensingcircuits (VTR. SENSE) 946 may also be selectively coupled to the rightatrial lead 804, coronary sinus lead 806, and the right ventricular lead808, through the switch 926 for detecting the presence of cardiacactivity in each of the four chambers of the heart. Accordingly, theatrial and ventricular sensing circuits 944 and 946 may includededicated sense amplifiers, multiplexed amplifiers, or sharedamplifiers. Switch 926 determines the “sensing polarity” of the cardiacsignal by selectively closing the appropriate switches, as is also knownin the art. In this way, the clinician may program the sensing polarityindependent of the stimulation polarity. The sensing circuits (e.g.,circuits 944 and 946) are optionally capable of obtaining informationindicative of tissue capture.

Each sensing circuit 944 and 946 preferably employs one or more lowpower, precision amplifiers with programmable gain, automatic gaincontrol, bandpass filtering, a threshold detection circuit, or somecombination of these components, to selectively sense the cardiac signalof interest. The automatic gain control enables the device 800 to dealeffectively with the difficult problem of sensing the low amplitudesignal characteristics of atrial or ventricular fibrillation.

The outputs of the atrial and ventricular sensing circuits 944 and 946are connected to the microcontroller 920, which, in turn, is able totrigger or inhibit the atrial and ventricular pulse generators 922 and924, respectively, in a demand fashion in response to the absence orpresence of cardiac activity in the appropriate chambers of the heart.Furthermore, as described herein, the microcontroller 920 is alsocapable of analyzing information output from the sensing circuits 944and 946, a data acquisition system 952, or both. This information may beused to determine or detect whether and to what degree tissue capturehas occurred and to program a pulse, or pulses, in response to suchdeterminations. The sensing circuits 944 and 946, in turn, receivecontrol signals over signal lines 948 and 950, respectively, from themicrocontroller 920 for purposes of controlling the gain, threshold,polarization charge removal circuitry (not shown), and the timing of anyblocking circuitry (not shown) coupled to the inputs of the sensingcircuits 944 and 946 as is known in the art.

For arrhythmia detection, the device 800 utilizes the atrial andventricular sensing circuits 944 and 946 to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. It should beappreciated that other components may be used to detect arrhythmiadepending on the system objectives. In reference to arrhythmias, as usedherein, “sensing” is reserved for the noting of an electrical signal orobtaining data (information), and “detection” is the processing(analysis) of these sensed signals and noting the presence of anarrhythmia.

Timing intervals between sensed events (e.g., P-waves, R-waves, anddepolarization signals associated with fibrillation) may be classifiedby the arrhythmia detector 934 of the microcontroller 920 by comparingthem to a predefined rate zone limit (e.g., bradycardia, normal, lowrate VT, high rate VT, and fibrillation rate zones) and various othercharacteristics (e.g., sudden onset, stability, physiologic sensors, andmorphology, etc.) in order to determine the type of remedial therapythat is needed (e.g., bradycardia pacing, anti-tachycardia pacing,cardioversion shocks or defibrillation shocks, collectively referred toas “tiered therapy”). Similar rules may be applied to the atrial channelto determine if there is an atrial tachyarrhythmia or atrialfibrillation with appropriate classification and intervention.

Cardiac signals or other signals may be applied to inputs of ananalog-to-digital (A/D) data acquisition system 952. The dataacquisition system 952 is configured (e.g., via signal line 956) toacquire intracardiac electrogram (“IEGM”) signals or other signals,convert the raw analog data into a digital signal, and store the digitalsignals for later processing, for telemetric transmission to an externaldevice 954, or both. For example, the data acquisition system 952 may becoupled to the right atrial lead 804, the coronary sinus lead 806, theright ventricular lead 808 and other leads through the switch 926 tosample cardiac signals across any pair of desired electrodes.

The data acquisition system 952 also may be coupled to receive signalsfrom other input devices. For example, the data acquisition system 952may sample signals from a physiologic sensor 970 or other componentsshown in FIG. 9 (connections not shown).

The microcontroller 920 is further coupled to a memory 960 by a suitabledata/address bus 962, wherein the programmable operating parameters usedby the microcontroller 920 are stored and modified, as required, inorder to customize the operation of the device 800 to suit the needs ofa particular patient. Such operating parameters define, for example,pacing pulse amplitude, pulse duration, electrode polarity, rate,sensitivity, automatic features, arrhythmia detection criteria, and theamplitude, waveshape and vector of each shocking pulse to be deliveredto the patient's heart H within each respective tier of therapy. Onefeature of the described embodiments is the ability to sense and store arelatively large amount of data (e.g., from the data acquisition system952), which data may then be used for subsequent analysis to guide theprogramming of the device 800.

Advantageously, the operating parameters of the implantable device 800may be non-invasively programmed into the memory 960 through a telemetrycircuit 964 in telemetric communication via communication link 966 withthe external device 954, such as a programmer, transtelephonictransceiver, a diagnostic system analyzer or some other device. Themicrocontroller 920 activates the telemetry circuit 964 with a controlsignal (e.g., via bus 968). The telemetry circuit 964 advantageouslyallows intracardiac electrograms and status information relating to theoperation of the device 800 (as contained in the microcontroller 920 ormemory 960) to be sent to the external device 954 through an establishedcommunication link 966.

The device 800 can further include one or more physiologic sensors 970.In some embodiments the device 800 may include a “rate-responsive”sensor that may provide, for example, information to aid in adjustmentof pacing stimulation rate according to the exercise state of thepatient. One or more physiologic sensors 970 (e.g., a pressure sensor)may further be used to detect changes in cardiac output, changes in thephysiological condition of the heart, or diurnal changes in activity(e.g., detecting sleep and wake states). Accordingly, themicrocontroller 920 responds by adjusting the various pacing parameters(such as rate, A-V Delay, V-V Delay, etc.) at which the atrial andventricular pulse generators 922 and 924 generate stimulation pulses.

While shown as being included within the device 800, it is to beunderstood that a physiologic sensor 970 may also be external to thedevice 800, yet still be implanted within or carried by the patient.Examples of physiologic sensors that may be implemented in conjunctionwith the device 800 include sensors that sense respiration rate, pH ofblood, ventricular gradient, oxygen saturation, blood pressure and soforth. Another sensor that may be used is one that detects activityvariance, wherein an activity sensor is monitored diurnally to detectthe low variance in the measurement corresponding to the sleep state.For a more detailed description of an activity variance sensor, thereader is directed to U.S. Pat. No. 5,476,483 (Bornzin et al.), whichpatent is hereby incorporated by reference.

The one or more physiologic sensors 970 may optionally include one ormore of components to help detect movement (via, e.g., a position sensoror an accelerometer) and minute ventilation (via an MV sensor) in thepatient. Signals generated by the position sensor and MV sensor may bepassed to the microcontroller 920 for analysis in determining whether toadjust the pacing rate, etc. The microcontroller 920 may thus monitorthe signals for indications of the patient's position and activitystatus, such as whether the patient is climbing up stairs or descendingdown stairs or whether the patient is sitting up after lying down.

The device 800 additionally includes a battery 976 that providesoperating power to all of the circuits shown in FIG. 9. For a device 800which employs shocking therapy, the battery 976 is capable of operatingat low current drains (e.g., preferably less than 10 μA) for longperiods of time, and is capable of providing high-current pulses (forcapacitor charging) when the patient requires a shock pulse (e.g.,preferably, in excess of 2 A, at voltages above 200 V, for periods of 10seconds or more). The battery 976 also desirably has a predictabledischarge characteristic so that elective replacement time can bedetected. Accordingly, the device 800 preferably employs lithium orother suitable battery technology.

The device 800 can further include magnet detection circuitry (notshown), coupled to the microcontroller 920, to detect when a magnet isplaced over the device 800. A magnet may be used by a clinician toperform various test functions of the device 800 and to signal themicrocontroller 920 that the external device 954 is in place to receivedata from or transmit data to the microcontroller 920 through thetelemetry circuit 964.

The device 800 further includes an impedance measuring circuit 978 thatis enabled by the microcontroller 920 via a control signal 980. Theknown uses for an impedance measuring circuit 978 include, but are notlimited to, lead impedance surveillance during the acute and chronicphases for proper performance, lead positioning or dislodgement;detecting operable electrodes and automatically switching to an operablepair if dislodgement occurs; measuring respiration or minuteventilation; measuring thoracic impedance for determining shockthresholds; detecting when the device 800 has been implanted; measuringstroke volume; and detecting the opening of heart valves, etc. Theimpedance measuring circuit 978 is advantageously coupled to the switch926 so that any desired electrode may be used.

In the case where the device 800 is intended to operate as animplantable cardioverter/defibrillator (ICD) device, it detects theoccurrence of an arrhythmia, and automatically applies an appropriatetherapy to the heart aimed at terminating the detected arrhythmia. Tothis end, the microcontroller 920 further controls a shocking circuit982 by way of a control signal 984. The shocking circuit 982 generatesshocking pulses of low (e.g., up to 0.5 J), moderate (e.g., 0.5 J to 10J), or high energy (e.g., 11 J to 40 J), as controlled by themicrocontroller 920. Such shocking pulses are applied to the patient'sheart H through, for example, two shocking electrodes and as shown inthis embodiment, selected from the left atrial coil electrode 826, theRV coil electrode 832 and the SVC coil electrode 834. As noted above,the housing 900 may act as an active electrode in combination with theRV coil electrode 832, as part of a split electrical vector using theSVC coil electrode 834 or the left atrial coil electrode 826 (i.e.,using the RV electrode as a common electrode), or in some otherarrangement.

Cardioversion level shocks are generally considered to be of low tomoderate energy level (so as to minimize pain felt by the patient), besynchronized with an R-wave, pertain to the treatment of tachycardia, orsome combination of the above. Defibrillation shocks are generally ofmoderate to high energy level (i.e., corresponding to thresholds in therange of 5 J to 40 J), delivered asynchronously (since R-waves may betoo disorganized), and pertaining to the treatment of fibrillation.Accordingly, the microcontroller 920 is capable of controlling thesynchronous or asynchronous delivery of the shocking pulses.

The filter circuits described herein may be implemented at or near oneor more of the components of FIG. 9. For example, a feedthroughcapacitor and/or a voltage divider circuit may be implanted at or nearthe connector, the switch 926, the sense circuits 944 and 946, or thepulse generator circuits 922 and 924.

Various modifications may be incorporated into the disclosed embodimentsbased on the teachings herein. For example, the structure andfunctionality taught herein may be incorporated into types of devicesother than the specific types of devices described above. In addition,different filtering components and filtering schemes may be employedconsistent with the teachings herein.

The various structures and functions described herein may beincorporated into a variety of apparatuses (e.g., a stimulation device,a lead, a monitoring device, etc.) and implemented in a variety of ways.Different embodiments of such an apparatus may include a variety ofhardware and software processing components. In some embodiments,hardware components such as processors, controllers, state machines,logic, or some combination of these components, may be used to implementthe described components or circuits.

In some embodiments, code including instructions (e.g., software,firmware, middleware, etc.) may be executed on one or more processingdevices to implement one or more of the described functions orcomponents. The code and associated components (e.g., data structuresand other components used by the code or used to execute the code) maybe stored in an appropriate data memory that is readable by a processingdevice (e.g., commonly referred to as a computer-readable medium).

Moreover, some of the operations described herein may be performed by adevice that is located externally with respect to the body of thepatient. For example, an implanted device may send raw data or processeddata to an external device that then performs the necessary processing.

The components and functions described herein may be connected orcoupled in many different ways. The manner in which this is done maydepend, in part, on whether and how the components are separated fromthe other components. In some embodiments some of the connections orcouplings represented by the lead lines in the drawings may be in anintegrated circuit, on a circuit board or implemented as discrete wiresor in other ways.

As used herein, terminology describing the coupling of components refersto any mechanism that allows signals to travel from one component toanother. Thus, coupling may be accomplished through use of an electricalconductor and/or an electrical component (e.g., an active or passiveelectrical circuit). In some cases two or more components may be“directly coupled.” That is, the components may be coupled via aconductor without any intervening components (e.g., an active or passiveelectrical circuit) between the components. Also, the term circuit isused herein in a broad sense of a component or components through whichcurrent may flow, and is not limited to the narrower definition of astructure that forms a loop. For example, a circuit may comprise onecomponent (e.g., a conductor, an electronic component, etc.) or morethan one component (e.g., several electronic components connected by oneor more conductors). As a specific example, a ground circuit may takethe form of a single conductor, a ground plane, multiple conductors,multiple ground planes, or some other form. As another specific example,an electrode circuit may take the form of a single conductor, aconductor and a connector, multiple conductors, or some other form.

The signals discussed herein may take various forms. For example, insome embodiments a signal may comprise electrical signals transmittedover a wire, light pulses transmitted through an optical medium such asan optical fiber or air, or RF waves transmitted through a medium suchas air, and so on. In addition, a plurality of signals may becollectively referred to as a signal herein. The signals discussed abovealso may take the form of data. For example, in some embodiments anapplication program may send a signal to another application program.Such a signal may be stored in a data memory.

Moreover, the recited order of the blocks in the processes disclosedherein is simply an example of a suitable approach. Thus, operationsassociated with such blocks may be rearranged while remaining within thescope of the present disclosure. Similarly, the accompanying methodclaims present operations in a sample order, and are not necessarilylimited to the specific order presented.

Also, it should be understood that any reference to elements hereinusing a designation such as “first,” “second,” and so forth does notgenerally limit the quantity or order of those elements. Rather, thesedesignations may be used herein as a convenient method of distinguishingbetween two or more different elements or instances of an element. Thus,a reference to first and second elements does not mean that only twoelements may be employed there or that the first element must precedethe second element in some manner. Also, unless stated otherwise a setof elements may comprise one or more elements.

While certain embodiments have been described above in detail and shownin the accompanying drawings, it is to be understood that suchembodiments are merely illustrative of and not restrictive of theteachings herein. In particular, it should be recognized that theteachings herein apply to a wide variety of apparatuses and methods. Itwill thus be recognized that various modifications may be made to theillustrated embodiments or other embodiments, without departing from thebroad scope thereof. In view of the above it will be understood that theteachings herein are intended to cover any changes, adaptations ormodifications which are within the scope of the disclosure.

What is claimed is:
 1. An implantable medical apparatus, comprising: afirst electrode circuit; a second electrode circuit; a feedthroughcapacitor coupled between the first electrode circuit and the secondelectrode circuit; a cardiac signal processing circuit comprising afirst terminal, a second terminal, and a ground circuit, wherein thesecond terminal is coupled to the second cardiac electrode circuit; anLC tank circuit coupled between the first electrode circuit and thefirst terminal of the cardiac signal processing circuit; a series LCcircuit coupled between the second electrode circuit and the firstterminal of the cardiac signal processing circuit; and a voltage dividercircuit coupled between the first terminal of the cardiac signalprocessing circuit and the second terminal of the cardiac signalprocessing circuit, wherein a center tap of the voltage divider circuitis coupled to the ground circuit.
 2. The implantable medical apparatusof claim 1, further comprising: a first diode coupled between the firstelectrode circuit and the ground circuit; and a second diode coupledbetween the second electrode circuit and the ground circuit.
 3. Theimplantable medical apparatus of claim 1, wherein: the voltage dividercircuit comprises a first capacitor and a second capacitor; the firstcapacitor is coupled between the first terminal of the cardiac signalprocessing circuit and the ground circuit; and the second capacitor iscoupled between the second terminal of the cardiac signal processingcircuit and the ground circuit.
 4. The implantable medical apparatus ofclaim 3, wherein the cardiac signal processing circuit comprises: afirst cardiac pacing circuit coupled between the first terminal of thecardiac signal processing circuit and the ground circuit; and a secondcardiac pacing circuit coupled between the second terminal of thecardiac signal processing circuit and the ground circuit.
 5. Theimplantable medical apparatus of claim 4, wherein the cardiac signalprocessing circuit further comprises a cardiac sensing circuit.
 6. Theimplantable medical apparatus of claim 3, wherein the feedthroughcapacitor has a capacitance of less than two nanofarads.